Liquid crystal display (LCD) projection systems offer the advantages of large image areas, compactness, ease of setup, light weight, and low cost compared with competing technologies. An LCD is comprised of thousands of small picture elements, or "pixels", which are either "on", "off", or "partially on". An image is displayed by the LCD by the appropriate manipulation, usually by applying an electric field, on the individual pixels. In the case of a twisted nematic (TN) LCD, if a particular pixel is "on", then the phase, and thus the polarization, of a linearly polarized light ray will remain unchanged as it passes through the pixel. However, if the pixel is "off", then the light ray will be rotated, i.e., its phase will be modulated so that its polarization angle is changed by 90 degrees. If the pixel is "partially on", then the light ray will be rotated by less than 90 degrees. An "on" pixel can be designated to represent either black or white. If the "on" pixel is designated as black, then the "off" pixel is designated as white, and vice versa. A "partially on" pixel represents a shade of gray. Polarizers are then provided on the LCD so that the polarization state of the light passing through the pixel is converted into the appropriate amount of transmission (black, white, or gray).
In the case of a super twisted nematic (STN) LCD, the optical effect arises from birefringence effects so that "on", "off", and "partially on" pixels each have a characteristic birefringence color. If the "blue mode" is used, the "off" pixel will have a blue color while the "on" pixel will be cream colored. If the "yellow mode" is used, the "off" pixel will be yellow and the "on" pixel will be blue-gray. A film may be added between the STN LCD and one of its polarizers to neutralize the color of the display, i.e., to convert the color display to a black and white display.
Current LCD projection systems typically employ forced air or liquid cooling to protect the temperature sensitive liquid crystal material. Even when using "hot" or "cold" mirrors to substantially remove the infrared component from the illumination, the approximately 55-60% of the visible light absorbed by the first of two conventional dichroic polarizer heats the LCD laminated thereto. In many LCD projectors in which the lamp, LCD, electronics, and projection optics are incorporated into one compact unit, the first polarizer is not laminated to the LCD to reduce conductive heating of the LCD, but the polarizer itself must still be cooled. Furthermore, all LCD projection systems suffer from low light utilization (color LCDs being typically 3-5% transmissive), thus usually necessitating brighter lamps to deliver good screen illumination. However, brighter lamps not only increase cost and power consumption, they also exacerbate the heating problem described above.